Thermal effects affect the rotation of asteroids

Hi there! Today we discuss the rotation of asteroids. You know, these small bodies are funny. When you are a big body, you are just attracted by your siblings. The Sun, the planets, etc. But when you are a small body, your life may be much more chaotic! Such small bodies not only experience the influence of gravitational perturbations, but also of thermal effects, especially when they are close enough to the Sun (Near-Earth Objects). Not only you have radiation pressure of the Sun, due to the electromagnetic field, but also a torque due to the difference of temperature between different areas of the surface of the small body.
Investigating such effects is particularly tough, since it depends on the shape of the asteroid, which could be anything. Shape, surface rugosity, thermal inertia… and the rotation state as well. When you face the Sun, you heat, but with a delay… and meanwhile, you do not face the Sun anymore… you see the nightmare for planetary scientists? Well, actually, you can say that it is not a nightmare, but something fascinating instead. You bypass such difficulties by making simplified models, and if you have the opportunity to compare with real data, i.e. observations, then you have a chance to validate your theory.
Today I present Systematic structure and sinks in the YORP effect, by Oleksiy Golubov and Daniel J. Scheeres. This study, published in The Astronomical Journal, tells us that sometimes the thermal effects may stabilize the rotational state of the asteroids.

Yarkovsky and YORP

As I said, the most important of the thermal effects, which are experienced by small asteroids (up to some 50 km), is the Yarkovsky effect. The area which faces the Sun heats, and then reemits photons while cooling. The reemission of these photons pushes the asteroids in a direction, which depends on the rotation of the body. As a consequence, this makes the prograde asteroids (rotation in the same direction as the orbit) spiral outward, while the retrograde ones spiral inward. The consequence on the orbits is a secular drift of the semimajor axis, which has been measured in some cases.
The first measurement dates back to 2003. The small asteroid (530 m) 6489 Golevka drifted by 15 km since 1991, with respect to the orbital predictions, which considered only the gravitational perturbations of the surrounding objects.
This effect had been predicted around 1900 by the Polish civil engineer Ivan Osipovich Yarkovsky.

And now: YORP. YORP stands for Yarkovsky-O’Keefe-Radzievskii-Paddack, i.e. 4 scientists. This is the thermal effect on the rotation. Most of the asteroids have irregular shapes, i.e. they do not look like ellipsoids, but rather like… anything else. Which means that the reemission of photons would not average to 0 over a rotational (or spin) period. As a consequence, if the asteroid is like a windmill, then its rotation will accelerate. Rotational data on Near-Earth Asteroids smaller than 50 km show an excess of fast rotators, with respect to larger bodies. And theoretical studies have shown that YORP could ultimately destroy an asteroid, in making it spin so fast that it would become unstable. The outcome would then be a binary object.

This is anyway a very-long-term effect.

YORP cycles

In fact, when the rotational energy is not high enough to provoke the disruption of the asteroid, the theory of YORP predicts that the rotational states experience cycles, over several hundreds of thousands years. During these cycles, the asteroid switches from a tumbling state, i.e. rotation around 3 axes to the rotation around one single axis, and then goes back to the tumbling states. These are the YORP cycles, which are not really observed given their long duration. But the authors of this study tell us that these cycles may be disrupted.

Normal and tangential YORP

The authors recall us that the YORP effect, which generates these cycles, is in fact the normal YORP. There is a tangential YORP as well. This tangential YORP (TYORP) is due to heat transfer effects on the surface, which results in asymmetric light emission. This yields an additional force, which alters the rotation.

New equilibriums in the rotational state

And the consequence is this: when you add the TYORP in simulating the rotational dynamics of your asteroid, you get equilibriums, i.e. rotational state, which would remain constant with respect to the time. In other words, under some circumstances, the rotational state leaves the YORP cycles, to remain locked in a given state. These states would have a principal rotation axis, which would be either parallel to the orbit, or orthogonal. In this last case, the rotation could either be prograde or retrograde.

Testing the prediction

This study suggests that the authors have predicted a rotation state. It would be good to be able to test this prediction, i.e. observe this rotation state among the asteroids.
The study does not mention any observable evidence of this theory. As the authors honestly say, this is only a first taste of the complicated theory of the YORP effect. Additional features should be considered, and the mechanism of trapping into these equilibriums is not investigated… or not yet.

Anyway, this is an original study, a new step to the full understanding of the YORP effect.

The study and its authors

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